I'm not sure about SWF, but some of the LFS keep their salinity down around 1.022 -1.023. Personally, I prefer to keep it up around 1.025 so that rules out most LFS saltwater for me. I just went with an inexpensive RO/DI unit.
It wouldn't even be too bad to get distilled water from the grocery store or LFS with a tank that size, but I agree that a cheapo RO/DI unit is much cheaper in the long run.